Temporal Variability of Ice Flow on Hofsjökull, Iceland, observed by ERS SAR Interferometry
Florian Mueller(1) and Helmut Rott(1)
University of Innsbruck,
In Iceland a significant number of glaciers reveals surge-type fluctuations of ice velocity. SAR interferometry offers an excellent possibility for studying the dynamic behaviour of these glaciers. Because of rapidly changing meteorological conditions and fast ice flow, only short repeat intervals, as provided by ERS 1/2 tandem SAR images, provide coherence suitable for InSAR analysis. As contribution to the ERS AO projects VECTRA and AO3.239 we exploited data of the ERS tandem archive to map ice flow on Hofsjökull ice cap in central Iceland that is know to have five surge-type outlet glaciers.
Several tandem pairs, acquired between October 1995 and February 1999, revealed sufficient coherence for ice motion retrievals. On two of the main outlet glaciers, Mùlajökull and Þjorsarjökull, significant temporal variations of ice velocities were observed. Because of the temporal variability, the topographic phase was calculated using a digital elevation model, kindly made available by the Science Institute of Iceland. Significant time intervals between ascending and descending tandem pairs prohibited also the combination of interferograms from different look directions. Therefore flow lines derived from Landsat 7 ETM+ were used to estimate the flow direction. This was only possible in ablation areas. In accumulation areas, where no distinct flow lines are visible, the surface parallel flow assumption was applied to estimate surface motion. For non-stationary glaciers this assumption may introduce major errors. Therefore line-of-sight motion is also presented.
On Múlajökull, an outlet glacier of Hofsjökull in the southeast, maps of ice motion were derived from four tandem interferograms acquired between October 1995 and February 1999. For highlighting temporal changes, detailed motion analysis was carried out at two transverse profiles. Along the profile at 1000 m a.s.l., just below the equilibrium line, an increase of surface velocity of about 80 % was observed between October 1995 and January 1996. On the terminus at about 850 m a.s.l. the velocity increased by about 40 %. Up to February 1999 the surface motion slowed down to values comparable to those observed in October 1995.
For Þjorsarjökull, a surge type outlet of Hofsjökull towards east, seven interferograms, acquired between October 1995 and February 1999, were analysed. In winter 1995/96 the glacier was in the final stage of a surge that had started in 1994. The motion of the lower part of the terminus decreased from 0.45 m/day in October 1995 to 0.05 m/day in January 1997. On the other hand, three ice flows, descending from the accumulation to the ablation area, accelerated by more than 50 % from October 1995 to March 1996, and slowed down later on. Overall, the InSAR analyses reveal significant spatial and temporal variability of ice motion on Hofsjökull. Of particular interest is the acceleration of ice flow in upper glacier areas coincident with post-surge slowing down on the terminus of Þjorsarjökull, which does not match the typical behaviour of surging glaciers.